The proposed research will examine in detail the role of poly (ADP-ribose) in DNA damage and DNA repair in human fibroblasts from normal, xeroderma pigmentosum patients, and parental heterozygotes. DNA damage will be induced by treatment with N-methyl-N'-nitro-N-nitrosoguanidine and UV-irradiation. A newly developed sensitive and selective chemical assay will be applied to measure the total intracellular levels of poly (ADP-ribose) following DNA damage. The size distribution of polymer chains and the intracellular levels of the precursor NAD will also be studied in order to estimate the rate of turnover of poly (ADP-ribose). The metabolism of poly (ADP-ribose) in vivo will be related to the activities of poly (ADP-ribose) polymerase and poly (ADP-ribose) glycohydrolase which will be measured independently in nucleotide permeable cells. All of the parameters of poly (ADP-ribose) metabolism will be quantitatively related to the amount of DNA damage using velocity sedimentation on alkaline sucrose density gradients and to the amount of unscheduled DNA synthesis using equilibrium sedimentation on alkaline cesium chloride density gradients. Since poly (ADP-ribose) is formed from NAD which is derived from the vitamin niacin, the results will be carefully examined for potentially significant nutritional factors which relate to environmental carcinogenesis.